Mass spectrometers suitable for the analysis of complex organic compounds are well known in the art and may incorporate mass filters of either the quadrupole or the magnetic sector type. The latter type may also incorporate an electrostatic filter in addition to the magnetic sector mass analyser, and this type of double focusing instrument is generally capable of higher mass resolution than a single focusing magnetic sector or quadrupole filter instrument.
Recently, instruments with more than one mass or momentum analysis filter have been constructed, and these have been found to have some advantages for the analysis of complex organic compounds, and especially mixtures of compounds. Instruments of this type commonly incorporate three quadrupole filters; a magnetic sector, electrostatic sector, and a quadrupole filter; two magnetic sectors and an electrostatic sector; or other combinations. The invention to be described is applicable to all these forms of spectrometers, and to any other form which utilises an ion source similar to those described.
An important problem frequently encountered in the mass spectrometric analysis of organic compounds is the ionization without excessive decomposition of thermally unstable or involatile samples. This is particularly important in the analysis of biochemical samples, because many important biochemicals fall into these categories. Many solutions to this problem have been proposed, including field ionization and desorption, atmospheric pressure ionization, electrohydrodynamic ionization, secondary ion mass spectrometry involving bombardment by ions, neutral particles, or laser radiation, etc., and many others. All these methods of ionization are capable of producing ions of very high mass from organic compounds, in some cases greater than 5000 daltons, which makes severe demands on the stability and calibration of the spectrometer mass analysing filters, especially at high resolution, and when accurate mass determinations are required. It has always been necessary to calibrate the mass scale of a mass spectrometer, usually by means of a reference compound which yields a spectrum consisting of peaks of accurately known mass, and this becomes very important if accurate mass measurements (as opposed to mass numbers) are wanted. When accurate mass measurements at very high mass are required, the selection of a suitable reference compound becomes one of the most difficult features of the analysis, especially because the reference compounds themselves tend to be difficult to ionize. In many cases the absence of a suitable high mass reference compound severely limits the applications of the "soft" ionization sources mentioned above.
A conventional calibration procedure suitable for accurate mass measurement of organic compounds at high resolution involves the simultaneous introduction of the sample and the reference compound into the ion source, so that the spectrum produced consists of peaks due to both compounds. The precise masses of the peaks due to the sample can then be calculated by interpolation between peaks of accurately known masses due to the reference compound. This can be done manually by examining a recording of the spectrum, or more usually by means of a computer programmed to perform the interpolation during a mass scan of the spectrometer, using the time of arrival of the peaks relative to the start of the scan as a measure of their mass. Alternatively, the technique of peak matching can be used. In peak matching, the spectrometer is switched rapidly and repetitively between a reference peak and a sample peak, and a narrow scan is made about the centre of each mass so that each peak can be displayed in turn on a long persistence oscilloscope. The ratio of the voltages required to superimpose the peaks can then be used to determine accurately the mass ratio of the two peaks. All these techniques are well known and need not be described further.
In the case of secondary ion organic mass spectrometry sources, such as those involving bombardment of the sample with a beam of ions, neutral particles, or light, etc, the simultaneous introduction of a reference compound and a sample can present a number of difficulties. For example, in the case of bombardment by neutral atoms of Argon of between 2 and 6 Kev energy, it has been found that the best results are obtained when the sample is dissolved in a medium of high viscosity and low volatility such as glycerol, and this solution is then coated on the target in the mass spectrometer source. If a reference compound is to be added to this solution it is essential that it too will dissolve in the medium chosen for the sample, and this can seriously limit the choice of reference componds. The high mass reference compounds most commonly used with conventional modes of ionization, i.e, triazine compounds and "Fomblin" (U.K. Registered Trade Mark) oil cannot be simultaneously introduced with a sample in neutral atom bombardment spectrometery because they are insoluble in glycerol and glycerol-like media. Consequently, very weak or even wholly indistinct spectra are obtained under these conditions. Sometimes it is possible to use the medium itself as a source of reference peaks, but these are usually limited to the low mass ranges. It is also sometimes possible to select a reference compound which is miscible with the medium chosen for the sample, but compounds selected in this way often have other disadvantages such as a relatively small number of useful peaks in the spectrum, and are usually only useful with a particular sample or class of sample. A further problem frequently arises from the apparent supression of the ionization of the sample by a reference compound dissolved in the same medium, or of the ionization of the reference compound by the sample. This effect is frequently observed in neutral particle and ion bombardment mass spectroscopies.
Similar problems of calibration are also encountered with field desorption ion sources, where a sample is coated on an emitter wire which is introduced into a field ionization mass spectrometer.
Attempts have been made to overcome these problems by the use of special targets which separate the sample and the reference compounds to prevent them from mixing. In one prior target assembly, two or more individual targets, one coated with the sample and another coated with the reference compound, are simultaneously bombarded by the primary beam, so that an ion beam containing ions from both the sample and the reference compounds is produced. This method suffers from the disadvantage that by its nature, the sample and the reference compound cannot both be positioned on the optical axis of the spectrometer, and usually both are displaced. This results in a serious loss in sensitivity on at least one of the samples, or, if a compromise position is adopted, a significant loss on both. In addition, with most practical forms of high resolution double focusing mass spectrometers, and especially with an instrument which is slightly out of adjustment, ions formed away from the optical axis of the instrument and which pass through the entrance slit of the analyser portion, can appear at an incorrect mass position in the spectrum. The use of a target of this kind can therefore result in the great majority of the ions being formed off the optical axis, so that the resultant peaks may appear at an incorrect mass position and a serious error is introduced.
An alternative known form of target consists of a rotatable shaft with two contiguous faces arranged in the manner of a wedge. The sample is coated on one face of the target, the reference on the other, and the shaft is positioned in the source so that one of the faces is in the correct position for conventional operation. In order to change from the sample to the reference compound, or v.v, the shaft is simply rotated through 180.degree.. This simple device suffers from the disadvantage that the two faces are not completely isolated from each other, so that mixing of the sample and reference compounds can occur whilst the shaft is in use. It is also difficult to operate automatically, especially quickly, because of the force exerted by the vacuum lock seals on the shaft of the probe and the presence of various guide rods and safety devices which are usually fitted to conventional insertion probes in order to ensure safe operation of the probe and to prevent it rotating in normal use. Further, it is often difficult to arrange mechanically, because it is necessary that the angled face of the target is correctly orientated towards both the primary beam and the optical axis of the spectrometer. This requirement largely determines the angle and position at which the shaft must enter the source housing, and the physical layout of many mass spectrometers very frequently precludes the use of a device of this kind without major modification of the source housing of the spectrometer.
It is an object of the present invention to provide apparatus for introducing samples and reference compounds into an ion or neutral particle bombardment source mass spectrometer in which these difficulties are overcome, and which can easily be fitted to most known types of spectrometer which have an insertion probe for the introduction of a sample coated on a target, thereby facilitating the accurate mass determination, especially at high mass, of the peaks in the spectrum of a sample, and in particular, a sample of high molecular weight of a compound of biological importance which is difficult to ionize by conventional methods.
It is another object of the invention to provide apparatus for introducing emitter wires loaded with sample or reference compounds into the source of a field ionization or field desorption mass spectrometer which operates in a similar manner.